It is known that various compounds can be used to treat cellulosic products such as cloth, yarn, paper, wood and wood substrates in order to impart thereto a flame-resistant quality. The use of wood treated with such compounds came into common usage during World War II, when it was used for the construction of blimp hangars.
Most known fire-retardant compositions make use of the same technical underpinning. These compositions comprise salt compositions, such as phosphates and sulfates, which becomes acidic under elevated temperatures. These salts can form, for example, phosphoric and sulfuric acid respectively, and other acidic phosphate and sulfate derivatives under high temperatures. The acid formed promotes charring of the treated material during exposure to fire. The resulting char reduces flammability by insulating the material from the fire, thereby reducing flame spread and penetration.
Many prior art compositions make use of active ingredients, such as ammonium sulfates, which have been found to be corrosive to metal, including any metal fasteners, such as staples or nails, used to secure the treated materials. Thus, for treatment of materials to be secured in place with metal fasteners, such as the majority of all building materials, compositions making use of less corrosive active ingredients is critical. However, many of the replacement materials, such as ammonium phosphate have high hygroscopicity (high moisture pick-up). Thus, there is a need for offsetting the hygroscopicity of the non-corrosive replacement salts.
Many of these compositions also include starches, such as molasses, in order to provide adequate charring. However, the use of such starches renders the treated material susceptible to degradation by adventitious organisms such as insects and molds.
Other fire-retardant compositions require a drying step with high heat after application to a material. This high heat step often leads to premature activation of the chemical processes designed to resist flame spread during fire. Thus, leading to acid hydrolysis of the cellulose fibers in the material treated.
Plywood sheaths are often treated with fire-retardant compositions and used as roof decking, where the temperature at the interface between the overlying shingles and the roof deck can often exceed 170.degree. F. However, many prior art fire-retardant compositions result in degradation of the materials to which they are applied at temperatures greatly below the temperature of combustion of the materials to which they are applied. Thus, it has been found that many roof decks constructed from plywood sheaths treated with prior art fire-retardant compositions begin to lose structural integrity through acid hydrolysis. This degradation occurs even more rapidly in warmer climates. Furthermore, the presence of moisture has been found to accelerate the rate of thermal degradation of many treated products. Thus, in many cases, the treated plywood used as roof decking required replacement in as little as two to seven years.
Many prior art fire-retardant compositions were found to discolor, as well as degrade, the material treated after prolonged exposure to the elements. For example, dark, reddish-brown charred spots soon begin to appear in wood products treated with prior art compositions. This discoloration prevents the use of the treated material where an exposed natural finish is aesthetically desirable. Moreover, the degradation of wood products used for structural purposes results in the products being unsuitable for use.
Organic compounds are often added to fire-retardant compositions to seal out moisture and also to lock the active salt ingredients into the structure of the treated material, thereby also raising the threshold temperature at which activation occurs. For example, U.S. Pat. No. 4,461,720and U.K. Patent Application GB 2,200,363 both disclose the use of melamine or derivatives thereof as binding resins or casings. These references disclose that melamine provides improved leach resistance. However, a major drawback associated with the use of melamine is its limited stability. Accordingly, storage of melamine is difficult, and any working stocks containing melamine have a very short working period before which they will set. Thus, melamine solutions must be replaced frequently, resulting in both increased handling and material costs.
There are a number of properties of natural wood that make it the product of choice for building construction. Its strength, appearance, durability, accessibility and non-corrosive nature make it ideally suited for building supports, framework and trims. However, wood is of course, highly flammable.
The treatment of wood with most prior art fire-retardant compositions has generally required the use of incising of the wood with small perforations in order to assure adequate penetration of the wood with the fire-retardant composition. However, this incising, by cutting through the fibers of the wood leads to a reduction in the structural integrity of the treated wood.
Due to the combustible nature of wood, building codes in virtually all U.S. municipalities restrict the use of untreated wood to certain applications. However, the fire-retardant treatment of wood has broadened the useful scope of both wood and wood substrates because these municipalities require the use of non-combustible materials, including treated wood, in applications where untreated wood would not be permitted. These municipalities rely on outside certification agencies to certify which fire-retardant treated materials meet the agency's criteria for strength, durability, fire-retardance and other properties. For example, all municipalities in western states rely on the International Conference of Building Officials (ICBO) to certify fire-retardant treated wood as suitable for all types of construction.